This article gives you an idea of some programming paradigms.

Procedural programming

A procedural programming language is one where programs are organized into procedures. Each procedure handles one particular task. The main function of the program then makes a series of calls to these procedures in order to archive its goal.

Object-Oriented Programming

Object-Oriented Programming (OOP) can be seen as an extension of procedural programming. It may be seen as the design of software using a collection of cooperating objects. In a traditional view a program may be seen as a collection of functions or simply as a list of code. Object-oriented programming is a method of programming, based on a hierarchy of classes and well-defined and cooperating objects. Classes and objects are the two main aspects of object-oriented programming.

Event driven programming

In event driven programming the program responds to events. The program flow is mainly determined by events such as mouse clicks, pressed keys, sensor information, etc. An event driven program has an event loop. In the event loop is a dispatcher that waits for an event to occur. The dispatcher then processes the event by calling the appropriate event handler.

GUI programming

A Graphical User Interface (GUI) is an interface which consists of graphical objects such as windows, menus and icons. They are used to interact with the underlying software.
Adding a GUI to a command line program means that your program uses an API of a GUI software. A GUI software can be a library, a RAD tool or an IDE. If you want to build GUI applications, you have to understand the principles of event driven programming and object-oriented programming.